Field of Invention
The present invention relates to a tensioner for engines, and more particularly to a tensioner for an engine with large and stable damping and minimum deflection of the shaft.
Description of Related Arts
Endless transmission device, such as belt drive or chain drive, is commonly used for transmitting power from one component to the other during the operation of engine. However, once the tooth profile of the endless transmission device is worn, slippage will occur. As a result, the endless transmission device not only losses its power transmitting ability but also can be noisy. If the slippage of the endless transmission device keeps occurring, the noise will become serious. This noise problem from the slippage of the endless transmission device is often the biggest problem for the driver of the vehicle in pursuit of silent. In addition, the slipping of the endless transmission device will also shorten the service life span thereof. The damaged endless transmission device will damage the engine as well. Therefore, a tensioner is required for incorporating with the endless transmission device to maintain the tension thereof. In other words, the belt drive or the chain drive can be remained in a stable tension state by the tensioner in order to prevent the belt drive or the chain drive from slipping or tooth-skipping.
In the current market, an automatic belt tensioner with a damping mechanism is able to solve the vibration and noise problems from the front assembly of the engine. However, most of the existing damping mechanism technologies have the common problems of lacking a damping force with the severe attenuation of the damping force.
Referring to FIGS. 6 to 9 of the drawings, these are the two common conventional designs utilized in the current market. The first conventional tensioner comprises a dish-shaped elastic member 25 being squeezed along its axis direction to be deformed by a pressing force, so as to generate an axial force to a damping member 26 and a cover 27, wherein the damping member 26 and the cover 27 are closely connected with each other to define a connection surface 28. Since the damping member 26 is made of wearable materials, and the cover 27 and the damping member 26 is rotated relatively during the operation of the tensioner, the connection surface 28 has friction coefficients. So a friction force, which is generated between the damping member 26 and the cover 27 during the relative rotation operation therebetween, is determined by the pressing force provided by the dish-shaped elastic member 25. In addition, the elastic member 25 is made of material with high rigidity, so that a small amount of compression of the elastic member 25 will generate a relatively large amount of axial force. In other words, even if the change of the compression displacement of the elastic member 25 is small, the change of the pressing force produced towards the axial direction will relatively large. Normally, the rigidity level of the elastic member is approximately 2000 N/mm, so that all relevant parts of the tensioner will be collected towards the axial direction. In addition, since the axial force becomes very large, the damping will be unstable. Furthermore, after the tensioner is used for a long time, a clearance between the damping member 26 and the cover 27 will become very large due to the friction force therebetween. Therefore, the compression displacement of the elastic member 25 will gradually decrease, and the damping force generated from the elastic member 25 will normally be dropped more than 50%. Therefore, large amount of deformation of the elastic member 25 is required in order to maintain enough frictional forces between the damping member 26 and the cover 27. However, the compression displacement of the elastic member of the tensioner is constant in the initial design, so the frictional forces between the damping member 26 and the cover 27 will gradually decrease during the operation of the tensioner. Accordingly, the tensioner cannot be designed to provide sufficiently large amount of friction forces. For example, in order to provide 500 N of frictional forces by the tensioners, the elastic member 25 must be designed to provide at least 1000 N of frictional forces. Conversely, if the elastic member 25 is designed to generate 500 N of frictional forces, the remaining frictional forces provided by the elastic member 25 will be decreased to 250 N or even lower after the tensioner is in use for a period of time. In other words, the biggest drawback of the above mentioned tensioner is that the frictional forces are dramatically decreased with use, so that the attenuation of damping is serious. Therefore, the tensioner must be maintained frequently in order to meet the dampening requirements.
Referring to FIG. 8 and FIG. 9 of the drawings, a second conventional tensioner is illustrated, wherein the second conventional tensioner comprises an elastic member 30 which is loaded after assembling. The elastic member 30 is loaded by means of pulling operation, wherein the elastic member 30 comprises a first hook end 32 and an opposed second hook end 33, wherein the first hook end 32 is attached with an annular damping member 29, and the second hook end 33 is attached with a tension arm 31. The annular damping member 29 is installed in an inner diameter of the elastic member 30, wherein an outer surface of the damping member 29 is contacted with the inner surface of the elastic member 30, and the inner surface of the damping member 29 is contacted with a top surface 34 of the tension arm 31, such that the frictional forces will be generated between the damping member 29 and the tension arm 31. In other words, the damping member 29 is located between the elastic member 30 and the tension arm 31. When the tensioner is operated, the damping member 29 is rotated with respect to the top surface 34 of the tension arm 31 to generate frictional forces, wherein the diameter of the elastic member 30 is gradually decreased during the rotation of the tension arm 31, so as to generate a pressing force to the damping member 29. So, the damping member 29 is closely contacted with the tension arm 31 by the pressing force, to provide sufficiently large amount of friction force between the damping member 29 and the tension arm 31. However, the above mentioned structure has the following disadvantages. Although the attenuation of the frictional forces is lower than the first conventional tensioner, the second conventional tensioner only can provide low power of frictional force. Since the positive frictional force only comes from one source, the ratio of the torque of the damping torque and torque of the elastic member cannot exceed 0.6, which cannot meet some of the large damping system requirements. Since the pressing forces applied on the damping member 29 is determined by the change of the inner diameter of the elastic member 30, the cost for manufacturing the elastic member 30 to provide large amount of damping forces will be increased correspondingly. In addition, the change of the inner diameter of the elastic element 30 is determined by the tensile of the elastic element 30. In other words, the change of the inner diameter of the elastic member 30 is determined by the tensile of the elastic member 30, and then the pressing forces applied on the damping member 29 are determined by the change of the inner diameter of the elastic member 30, so the damping member 29 cannot provide large amounts of frictional forces. Therefore, it is preferred that the pressing forces applied on the damping member 29 is directly determined by the tensile of the elastic member 30, so as to minimize the energy loss during the transformation process. Conversely, the tensile for the damping member 29 is amplified via the change of the inner diameter of the elastic member 30 as well as that the damping member 29 is worn. In other words, the dimension change of the damping member 29 due to the wear and tear thereof is made up by the dimension change of the inner diameter of the elastic member 30. However, the change of the inner diameter of the elastic member 30 and the tensile thereof are mutually corresponding. So, it is preferred that the elastic member 30 should have small change of the inner diameter and large tensile. As a result, the wear and tear of the damping member 29 has stronger effect to the frictional forces. In other words, the frictional forces are decreased after the tensioner is operated during a period of time, and the attenuation of the friction force is still large.